An electrochemical gas sensor for sensing an oxidisible or reducible gas (e.g. carbon monoxide) in the atmosphere usually contains a sensing or working electrode, a counter electrode and an inlet (usually a diffusion barrier) to allow the atmosphere to permeate to the sensing electrode. Both electrodes are in contact with an electrolyte in order firstly to produce an electro chemical reaction at the sensing electrode with the gas to be sensed, and secondly to produce an electrochemical reaction at the counter electrode with oxygen in the atmosphere, electrolyte or other gas source. Current is carried through the solution by ions produced in the reaction and by electrons through an external circuit, the current in the circuit indicating the gas concentration. A reference electrode may be employed in combination with a potentiostat circuit to maintain the potential between the sensing electrode and the cell electrolyte in order to increase stability of operation.
In terms of physical construction, the sensor normally comprises an external housing which acts as a reservoir for the electrolyte, a wick or matrix to hold the electrolyte in contact with the electrodes, and external electrical terminals making electrical connection with the electrodes.
The majority of present sensor cells use a stacked electrode arrangement, as for example in U.S. Pat. No. 4,406,770, with the electrodes held close together in contact with wick pieces by pressure from an O-ring compression seal. This design has the disadvantage that a large number of components are typically needed as electrode contacts, to separate the electrodes, and to provide wick access for the electrolyte from the reservoir to the electrodes. This leads to high assembly time and cost. A second disadvantage is that the seal will tend to relax with time, leading to leakage of the electrolyte and cell failure. This second disadvantage is avoided by designs in which the components are sealed together as for example in German patent document DE 33 24 682, in which stacked electrodes are held in position on a central body by a heat and pressure welding operation. Both cells of the types in U.S. Pat. No. 4,406,770 and German patent document DE 33 24 682 use metal strip electrode contacts which, owing to the corrosive nature of the electrolyte, have to be of platinum or similar noble metal, and so are a significant fraction of the cell material cost.
European patent document EP-A-0 461 449 discloses an arrangement for preventing elecrolyte leakage in a gas sensor wherein a support body of silver is formed with a central annular reservoir for electrolyte with a narrow top region with apertures therethrough leading to a gold electrode layer applied to the top surface of the support body. The coating is configured as an adherent joiner to the support body so that it forms an impermeable adherence region in the lateral direction between coating and support body to prevent permeation of the electrolyte throughout the gold electrode. An external terminal lead is connected to one edge of the gold electrode layer. Whilst this construction prevents electrolyte leakage, it is specific to the materials described and does not disclose a means for general application.
U.S. Pat. No. 5,183,550 discloses a gas sensor in which the sensing, counter and reference electrodes are mounted in a common plane on a common ceramic substrate, with contact leads extending from the electrodes to the other surface of the substrate for electrical connection. However, the resulting sensor still has many components and involves relatively complex manufacturing steps which will add to manufacturing cost.